Liquid level indicating means



Nov. 2l, 11950 w. H. KLIEVER LIQUID LEvEL INDICATING mms Filed June 25, 1945 NQ m 4 Patented Nov. 21, 1950 UNITED STATES PATENT OFFICE LIQUID LEVEL INDICATING MEANS Y Waldo ll. Kliever, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application June 25, 1945, Serial No. 601,367

11 claims. (ci sis-2s) This invention relates to the eld of electric measuring apparatus, and more particularly to devices of this nature in which an electrical circuit element is arranged for variation with variation in a condition to be measured, and change in l,

the condition is measured by measuring the related change in the circuit element.

The invention comprises an improvement on the instrument disclosed in the copending application Serial No. 579,253 of Curtis R. Schafer, illed February 22, 1945, now forfeited, and assigned to the assignee of the present application.

It is an object of this invention to providean improved apparatus of the type disclosed in the copendlng application, wherein the indication of a measuring instrument including an electronic rectiiler is made relatively insensitive to variations in the characteristics of the rectifier.

It'is another object of the invention to provide an improved measuring system of the type disclosed in the copending application which is relatively insensitivel to variation in an input voltage supplied to a condition responsive member.

It is a further object of the invention to provide an improved measuring instrument including "a source of input voltage and an electronic rectier, in which the indication of the system is relatively unaffected by change in the input voltage and by change in the characteristics of the electronic rectifier.

Various other objects, advantages, and features of novelty which characterize my invention are pointed out with particularity in the claims annexed herein kand forming a part hereof. However, for a better understanding of the invention. its advantages, and objects attained by its use, reference should be had to the subioined drawing, whichforms a further part hereof, and to the accompanying descriptive matter, in which I have illustrated and described a preferred embodiment of my invention. Iny the drawing,

Figure 1 is a schematic wiring diagram of a preferred embodiment of my invention, and

Figures 2, 3, 4, and 5 are graphical representations of voltage relationships prevailing in my invention.

Figure l shows the system as energized from suitable source III of voltage of a power frequency.

The system is shown to comprise a high fre-y quency oscillator II deriving its plate and iliament energy from a power supply I2 and supplied with modulating voltage from source I0 from a. transformer I3. The oscillator energizes an impedance bridge I4, one of whose arms comoscillator II through a. transformer 2| is im pressed'bv means of coupling capacitor 22. The rectified or demodulated voltages supplied by the diodes are passed through filters 23 and 24 and 4then through a variably .tapped resistor 25, the latter voltage being impressed across the whole resistance winding, and the former voltage through a circuit including a portion of the winding, so that for any relation between the rvoltages there is a position of the tap at which no current flows in the diode output circuit influenced by unbalance of the bridge. The circuit referred to is coupled. by means of -a transformer 26, to an amplifier 21 which derives its power from source Ill, and is effective to control the energization of a motor 3B. Operation of the motor is effective to adjust the tap referred to, and also to determine the indication of an indicator 3|. The structure of these main components of my system will now be gone into in such further detail as is necessary.

Oscillator II is shown as having a pair of iliarnent voltage input terminals 34 and 35, a pair of plate voltage input terminals 32 and 33, a pair of modulating voltage input terminals 36 and 31, and a pair of signal output terminals 40 and 4I. The oscillator may be oi' any suitable type generting a signal of communications frequencysuch, for example, as kilocycles--and so arranged as to make possible the modulation of this relatively high frequency voltage at a low or power frequency-such for example as 400 cycles. Such oscillators are well known in the art, and the details of the internal structure of the oscillator are therefore not included in this application. A circuit diagram forsuch an oscillator is given in Figure 1937 on page 412 of the Radio Amateurs Handbook, 1944 edition. Y

The construction of power supply I2 is also the conventional arrangement by which there is derived from the power line an alternating voltage of small amplitude for filament heating and a unidirectional voltage of practically constant magnitude for plate energization, the former customarily being ,obtained from a Ifilament v`I2 is energized from source III through conductors 50, 5I, and 53 and conductors 54, 55, and 5l. Plate voltage is supplied to oscillator II from power supply I2 throughconductor's `60 and 6I. Filament current is provided to oscillator II from power supply I2 through conductors 62 and 63 and conductors 64 and 55. I

Transformer I3 isfshown to comprise a primary windingy B6 and asecondary winding 61.

associo The function of this transformer is to provide voltage for modulating the high frequency output of oscillator I'I at the lower frequency of source I8, and to that end primary winding 88 is connected to source I8 through conductors 88, 8|, and 18 and conductors 84, 58, and 1|, while secondary winding 61 is connected to terminals 88 and 81 of oscillator by means of conductors 12 and 18, respectively.

I have shown my measuring instrument as embodied in a device for indicating the amount of iluid in a container. responsive device |8 is shown as comprising a pair of electrodes 88 and 88 insulatingly supported in a container 18 for a dielectric fluid 18.

With rise and fall of fluid in container 18, the

uid replaces air to a varying degree as the di electric of the capacitor 88 comprising plates 88 and 88 and therefore the capacitance of the capacitor is varied in accordance with change in the amount of iluid in the container.

Bridge I4 is shown to comprise a pair of input terminals 14 and 15 and a pair of output terminals 18 and 11. A load resistor 18 is connected across output terminals 18 and 11. The ratio arms of the bridge are shown as comprising a For this purpose, condition pair of impedance members 88 and 8| located in adjacent arms of the bridge; and capacitor 88 comprised in the third arm of the bridge is shown as being compared with a standard capacitor 82 located in the fourth arm of the bridge. The bridge is energized with the modulated output voltage of oscillator I through conductors 88 and 84 which connect terminals 48 and 4|, respectively, of the oso.1lator with input terminals 14 and 15 of the bridge.

Full wave rectier tube I1 is shown to comprise a heater lament 88, a cathode 88. and a pair of anodes 81 and 88, allcontained within an evacuated envelope 8|. Filament 88 is energized from terminals 48 and 41 of power supply |2. Functionally, the tube is divided into a ilrst j diode detector 82 and a second diode detector 88. The output of bridge I4 is applied to diode detector 82, output terminal 11 being connected with cathode 88 by conductor 84, while output terminal 18 is connected with anode 88 by a conductor 85, coupling capacitor 28, and conductors Transformer 2| is shown to comprise a primary winding |8| and a secondary |82. The function of this transformer is to energize diode 88 from oscillator and to this end primary winding |8| is connected with the conductors 88 and 84, while secondary winding |82 is connected between cathode 88 and anode 81 by conductor |88, condenser 22, and conductors |81 and and by conductors |88 and |88.

For reasons which will presently be disclosed. nlter circuits 28 and 24 are connected in parallel with diodes 82 and 88. Filter circuit 28 may ybe traced from cathode 88 through conductors |88, ||2, and H8, a capacitor ||4, conductor ||8, an inductor ||1, and conductor |88to plate 88. Filter circuit 24 may be traced from cathode 88 through conductors |88, I I2, and |2|, a capacitor |22, conductors |28 and |24, an inductor |28I and conductor y| to plate 81.

Potential divider 28' is shown to comprise a resistance winding |21 and a sliding contactar |88, the latter being actuated mechanically from motor 88 through a link |8| as will presently be described. Transformer 28 is shown as comprising a primary winding |82 and a secondary winding |88. 'I'he voltage drop across capacitor |22 is impressed across winding |21 of potential divider 28 by conductors |84 and |88. Slider |88 is connected to one terminal of transformer primary |82 so that the primary winding is oonnected in series with capacitor ||4 and a variable portion of resistance winding |21, depending upon the position of slider |88, through a circuit including conductors |88, ||8, ||8, and |81.

It is apparent that winding |21 comprises a load circuit for diode 88, and that primary winding |82 and the portion of winding |21 below slider |88 comprise a load circuit for diode 82. These load circuits have as a common portion cathode 88, conductors |88. ||2, and |85, and a variable portion of winding |21 depending on the position of slider |88. The current in the load circuit including primary winding |82 is made use ot electromagnetically in transformer 28 as a measure of the need for operation of motor 88.

Amplifier 21 may be 'any suitable ampliner which is effective to control the operation of the motor in a forward or reverse direction in accordance with reversal of the phase of a signal voltage impressed upon the amplifier. Such amplifiers are known in the art (see Anschutz-Kaempfe 1,586,233, for example), and the detailed construction of this amplifier will therefore not be given. Amplifier 21 is shown as comprising a pair of power input terminals |48 and |4|, a pair of signal input terminals |42 and |48. and a plurality of power output terminals |44, |48, |48, and |41. The signal input to amplifier 2l is obtained from secondary winding |88 of transformer 28 through conductors |58 and |5| which are connected respectivehr to input terminals |42 and |48, and power for energizing amplifier 21 is derived from source |8 through conductors 58 and |52 and conductors 54 and |58, completing the circuit between source I8 and input terminals |48 and |4| of amplifier 21.

Motor 88 may be of any suitable type adapted to cooperate with amplifier 21 for reversible energization thereby. The internal mechanical relations between the stator and rotor of this motor are therefore not shown, but the shaft |84 of the motor is shown as connected by link |8| with slider |88 of potential divider 25, and by link |85 with indicator 8|. Such reductiongearing as is desired may beV provided between motor 88 and links |8| and |85.

Indicator 8| is shown to comprise an index member |88 arranged for movement with respect to a graduated scale |81 in response to the actuation of link by shaft |54 of motor 88. The motor is shown as being energized from amplifier 21 through conductors |88, |8|, |82, and |88 connected respectively to terminals |44, |48, |48. and |41 of the amplifier.

By way of illustration I append a tabulation of the circuit constants of one successful embodimentv of my invention: operation of the invention is not, however, to be considered as 'limited to any single set of speciiicvalues for the-circuit components thereof.

Source |8 115 volts 400 cycles Oscillator kllocycles 100% Y modulated at 400 cycles Bridge input voltage 50 volts Condenser 82 50 micromicrofarads Condenser 22 100 micromicrofarads Condenser 28 100 micromicrofarads Condenser |22 .04 microfarad Condenser ||4 .04 microfarad assaeio Potential divider 10,000 ohms Resistor 18 '.1 megohm Tube used 'IE6 Transformer 28 1 Audio frequency 1:1 ratio Transformer 2| Radio frequency 1:1

` ratio Operation The operation of my invention will be best understood if reference is made to Figures 2, 3. 4, and 5. On energization of the system from source I0, oscillator is energized through power supply |2 and modulation transformer I3, and impresses upon the inputterminals 14 and I5 .of bridge I4 an alternating voltage having the wave shape shown in curve |10 of Figure 2. It will be seen that thisvoltage alternates about a zero value indicated by line |1| at the frequency of oscillator and that the amplitude of each alternation varies, due to the eiiect of transformer |3, in such a fashion that the envelope of the wave train, indicated by the lines |13 and |l4,-corresponds to the wave form of the modulating voltage supplied by source I0.`

As is well known to those skilled in the art, the nature of bridge |4 is such that, so long as the ratio of the impedance of capacitor 69 to that of standard capacitor 82 is the same as the ratio between the impedances 8| and tu, no output voltage/appears across load resistor 18.. If, on the other hand, the first ratio is not equal to the second ratio, a voltage having the wave shape and frequency indicated by curve |10 appears across load resistor i8, the voltages when the first ratio is greater and when it is less than the second ratio being displaced in phase by 180 degrees of the carrier voltage. In my invention, however, I do not propose to use the bridge so broadly, in that I do not contemplate making use of the phase reversal properties of the bridge. I propose simply to adjust the bridge so that it is in `balance when the container is emptyof,

fluid. Unbalance of the bridge due to change in the impedance of capacitor 89 as the fluid level rises then results in a bridge output voltage which, although it varies in amplitude, does not vary in its time-phase relationship regardless Aof what change takes place in the amount of fluid in tank 1l. Figure 2, therefore, is also illustrative of the wave shape of the output voltage derived from bridge I4, if it is understood that the maximum amplitude of curve |10, indicated at |12, varies with variation in the amount of uid in the tank.

.The bridge output voltage is impressed between the plate arid cathode of diode 92, which com-- prises a low resistance path in parallel with the bridge output resistor whenever anode 80 of the diode is positive, and functions substantially as an infinite resistance whenever the anode is negative with respect to the cathode. vSince limited power is available from the voltage` drop across resistor 18, the voltage drop between cathode 88 and anode L80 of diode 92 falls oipractically to zero during those half cycles kof the high frequency v.voltage when the plate is positive. The

wave shape of the voltage. actually existing be- I tween cathode and anode of the diode is therev,fore illustrated in Figure 3, the voltage following theinterrupted high frequency curve |80, whose upper envelope is shown at/I83, and whose lower envelope, not shown to avoidcomplicating the drawing. is substantially astraight line coincident with the aero value of voltage. It is realized that in practical applications of electron tubes, non-linearity and other peculiarities of the tube characteristics have a minor distorting effect uponthe lower envelope of )this voltage curve, but for all practical purposes the relationships Just recited hold true.

Curve |80 of Figure 3 may be analyzed into two principal components, shownln curves |84 and |85 of Figura/4: componentsof other frequencies may be found in curve |00, but they comprise so slight a proportion ofthe total voltage as to be negligible. It may therefore be assumed that across nlter 23 there is impressed a pair of voltages such as is shown in Figure 4.

The impedance of .capacitor ||4 to a voltage having the frequency of curve |85 is low, while its impedance to a voltage having thefrequency of curve |84 is high. 'I'he voltage drop across capacitor ||4 therefore has a major component of the frequency of curve |84, which it kwill be recalled is also the frequency of source I0, and the voltage tends to cause a flow of current from condenser ||4 through primary winding |32 of transformer 26, tap |30, and the lower portion of winding |21 of potential divider 25.

The operation of diode 93 is similar to that just recited for diode 82, and will not be illustrated step by step as with the operation of diode 82. Alternating voltage derived from oscillator through transformer 2| is impressed between cathode 86 and anode 81 and the resulting voltage drop between the cathode and anode has the same wave shape as that across diode 82. The voltage drop across capacitor |22 is therefore also predominantly of the frequency of source I0, and this voltage is impressed across the whole length of winding 21 of potential divider 25. This voltage remains of constant amplitude regardless of change in the balance condition of the bridge, and serves as a comparison voltage for the output of diode 92. If the voltage of oscillator should change, the voltages across both diodes change in proportion. The eect of such voltage variation is therefore minimized.

Neglecting such phase shift as may be introduced by the apparatusused, and which may be corrected for by suitable conventional methods, it will be appreciated that the voltage drop across condenser |22 is either in phase with that across condenser ||4 or 180 degrees out of phase therewith, depending upon the mode of connection of transformers |3 and 2| and their respective cir-l cuits. For a proper operation of my invention, the transformers must be so connected that for any particular instantaneous value of source I0 the potential increases in a single direction from conductor |35 whether it be measured along the wholelength of winding |21 or through a p0rtion of winding |21 and then through slider |30 andprimary winding |32 of transformer 26. The voltages supplied by oscillator and transformer l2| are so chosen that when the capacitance of capacitor 68 is greatest, corresponding to a full container, the voltage drop across condenser ||4 is equal to that across the entire resistance winding |21. The two voltages, being in phase and arranged in opposition, buck one another out so that no current flows iny primary winding |32. This is best shown in Figure5, where curve |80 is the voltage across the entire resistance winding |21, curve |8| is that across the lower portion of the windingr as determined by the position of tap |30, andcurve |82 is that across condenser andere iii. The system is in balance when curves III -and|l2areoppositeinphaseandequalinam be opposed by a larger and larger portion of that across the resistance winding until a value of the latter is found which is just equal to the voltage across the condenser. The flow of current in primary winding |32 then ceases.

If the level of liquid in the tank falls. the voltage across condenser ||4 falls oi! as compared to that across the portion of the resistance winding determined by the position of slider ill, and current again ows in the primary winding |32. This current is of the opposite phase to that nrst described, but its flow can also be caused to cease by movement of slider |3|| downward until the voltages again are equal.

The'current in winding |32 having the first phase just recited is effective to energize motor Il to operate in a ilrst direction, causing indicator 3| to move toward full, and simultaneously causing slider |3|| to move upward along resistance winding |21 until the voltages are equal. Similarly, current of` the second phase flowing in primary winding |32 is eifective to energize motor 30 to operate in the opposite direction, causing indicator 3| to move toward empty. and simultaneously causing slider |30 to move downward along resistance winding |21 until the voltages are again equal.

Numerous objects and advantages of my inven tion have been set forth in the foregoing description together with details of the structure and function of the invention, and the -novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and I may make changes in detail, especially in matter of shape, size and arrangement of parts', within the principle of the invention, to the full extent indicated bythe broad kgeneral meaning of the terms in which the appended claims are expressed.

I claim as my invention:

l. A device of the class described comprising, in combination: a source of alternating voltage of a signal frequency modulated at a power frequency; an impedance bridge; means energizing said bridge from said source; means varying the balance of said bridge in response to a condition, whereby to vary the amplitude of the voltage output of said bridge; an asymmetrically conducting' member comprising an velectron discharge device including an electron-emissive cathode and a plurality of anodes; coupling means impressing said output voltage between said cathode and one of said anodes, whereby to cause the flow of a first pulsating unidirectional current through .said member; means deriving from said pulsating current a voltage component of said power frequency, the amplitude of said component varying with the degree of unbalance of said bridge; further coupling means mpressing voltage from said source between said cathode and another of said anodes, whereby to cause the flow of a second pulsating unidirectional current through said member; means deriving from said second pulsating current a second voltage component of said power frequency and of constant amplitude;

l Y means, including a variably tapped resistor, whereby upon adjustment of said tap said iirlt voltage component may be opposed by on equal and opposite voltage comprising a portion of said second voltage component; and means responsive to now of current in said last means.

2. A device of the class described comprising, in combination: a source of alternating voltage oi' a signal frequency modulated at a power frequency; an impedance bridge; means energizing said bridge from said source; means varying the balance of said bridge in response to a condition. whereby to vary the amplitude of the voltage output of said bridge; an asymmetrically conducting member comprising an electron discharge device including an electron-emissive cathode and a plurality of anodes, coupling means imp said output voltage between said cathode and one of said anodes, whereby to cause the ilow of a iirst pulsating unidirectional current through said member; means deriving from said pulsating current a voltage component of said power frequency, the amplitude of said component varying with the degree of unbalance of said bridge; further coupling means impressing voltage fromsaid source between said cathode and another of said anodes, whereby to cause the flow of a second pulsating unidirectional current through said member; means deriving from said second pulsating current a second voltage component of said power frequency and of constant amplitude; means, including a variably tapped resistor, whereby upon adjustment of said tap said first voltage component may be opposed by an equal and opposite voltage comprising a portion of said second voltage component; a motor; and means energizing said motor in accordance with the phase of the preponderant one of said opposed components.

3. A device o! the class described comprising. in combination: a source of alternating voltage oi a signal frequency modulated at a power frequency; an impedance bridge; means energizing said bridge from said source; means varying thi balance of said bridge in response to a condition whereby to vary the amplitude of the voltage output of said bridge; an asymmetrically conducting member comprising an electron discharge `device including an electron-emissive cathode and a plurality of anodes; coupling means impressing said output voltage between said cathode and one of said anodes, whereby to cause the flow of a first pulsating unidirectional current through said member; means deriving from said pulsating current a voltage component of said power frequency, the amplitude of said component varying with the degree of unbalance of said bridge; further coupling means impressing voltage from said source between said cathode and another of said anodes, whereby to cause the iiow of a lecond pulsating unidirectional current through said member; means deriving from said second pulsating current a second voltage component of said power frequency and of constant amplitude; means, including a variably tapped resistor. whereby yupon adjustment of said tap said tlrst voltage component may be opposed by an equal and opposite voltage comprising a portion of said second voltage component; means energizing nid motor in accordance'with the phase of the pre ponderant one of said `opposed components; and means actuated by said motor for operating said tap to varythe portion of said second component opposing said nrst component.

4. A device of the class described comprising,

amera in combination: a source of alternating voltage of a signal frequency modulatedat a power frequency; means `varying the amplitude of mod u'- lated voltage from said 'source inl response to Va.

condition; demodulating meansenergiaed `with voltagesv from said lastl named means andfrom` said source; means connecting vol e drops across said demodulating' means in opposition, said means comprising a low-pass 'lter energized by each said voltage drop, a resistor having an adjustable tap, means impressingv the output of one of said filters across said resistor,y and means comparing the output of the other of said filters with the voltage across a variable portion of said resistor determined by the position of said tap, whereby upon adjustment of said tap the voltage supplied by said last named filter may be opposed by an equaland opposite voltage comprising a portion of the voltage supplied by the` other of said filters; and means responsive to the relation between said opposed voltages. 4

5. A device of the class described comprising, in combination: a source of lalternating voltage of a signal frequency modulated at a power frequency; means varying the amplitude of modulated voltage from said source in response to a condition; demodulating means energized with voltages from said last named means and from said source; means connecting voltage drops acrossdsaid demodulating means in opposition, said means comprising la low-pass filter energized by each said voltage drop, a resistor having an adjustable tap, means impressing the output of one of said filters across saidl resistor, and means comparingA the output of theother said filters with the voltage across a variable portion of said resistor determined by the position of said tap, whereby upon adjustment of said tap the voltage supplied by said last named filter may be opposed by an equal and opposite voltage comprising a portion of the voltage supplied by the other of said filters; a motor; and means energizing said motor in accordance with the relation between said opposed voltages. l

6. A device of the' class described comprising, in combination: a source of alternating voltage of a signal frequency modulated at a power frequency; means varying the amplitude of modulated voltage from said'source in response to a cluding condition responsive means for affecting the balance thereof; a 'voltage balancing network including means for adjusting the same: means energizing said bridge from said second source; asymmetrically conducting means applying voltages to said network from said second source yand from said bridge; a reversible motor; an indicator; means connecting said motor in driving relation to said indicator and to said nets source and any unbalance of said voltage comparing network. so as to rebalance the latter.

8. A device of the class described comprising, in combination: a source Vof alternating voltage of a carrier frequency modulated at a power frequency; a measuring circuit and a comparison circuit energized from said source, said measuring circuit comprising a normally balanced bridge, a first demodulator, and a first filter network, and said comparison circuit comprising a second demodulator and a second filter network; condition responsive means varying the state of balanceoi' said bridge whereby to vary the voltage supplied by said filter in said measuring circuit, an adjustable voltage balancing network; means energizing said network from said measuring circuit and said comparison circuit; a motor; means energizing said motor in accordance with a characteristic of any unbalance one of said filters across said resistor, and means comparing the output of the other of said filters with the voltage across a variable portion of said resistor determined by the position of said tap,

whereby upon adjustment of said tap the voltf age supplied byl said lastnamed filter may be opposed by an equal and opposite voltage comprising a portion of the voltage'supplied yby the other of said'iilters; means energizing said motor in accordance with the relation between said opposed voltages; and means actuated by said motor for adjusting said tap to yary the voltage opposing the output of said last named filter.

7. A device of the class described comprising, in combination: a rst source of alternating volt` ageof a power frequency; a second source of alternating voltage of a carrier frequency mod` ulated by said rst named voltage; a bridge involtage divider;

voltage appearing in said network; and means actuated by said motor to adjust said network so as to restore a condition of balance therein.

9. In a. device of the class described, in combination: a first source of alternating voltage, of varying amplitude, including a. carrier frequency component modulated at a power fre-v quency component; a second source of periodic voltage, of fixed amplitude, including at least a component of said power frequency; a voltage divider; means, including an asymmetrically conducting member, applying voltage of said power frequency from said second source across said means, including an asymmetrically conducting member, for deriving from said first source a voltage of said power frequency related in magnitude to the amplitude of said first periodic voltage; and power frequency means' responsive to inequality Vbetween said derived voltage and the voltage drop in a selected portion of said unit due to said first applied voltage.

l0. Apparatus employing voltage alternating at a relatively high frequency for controlling the energization of a device operable by voltage alternating at a relatively low frequency comprising, in combination: first and second sources supplying alternating voltages of a carrier frequency modulated at a powerfrequency; means varying tle output voltage supplied by one of said sources in accordance with the value of a condition;

demodulatingmeans; means, including connec-` velopes of the voltages supplied by said sources;

and `means connecting said output voltages in phase opposition to produce a resultantvoltage oi'power frequency having one phase or the other depending on the relative magnitude of said out-L` andere ternating at a relatively low frequency comprising, in combination: first and second sources supplying alternating `voltages of aV carrier frequency modulated at a power frequency; means varying the output voltage supplied by one of said sources in accordance with the value of a condition; demodulating means; means, including connections between said dexnodulating means and said sources, for producing output voltages of said power frequency in accordance with the envelopes of the voltages' supplied by said sources; and means connecting said output voltages in phase opposition to produce a resultant voltage of power frequency having one phase or the other depending on the relative magnitude of said output voltages, and means for energizing a device operable by voltage of said power frequency in accordance with the phase of said resultant voltage.

12. Apparatus employing voltage alternating at a relatively high frequency for controlling the energization of a device operable by voltage alternating at a relatively low frequency comprising, in combination: first and second sources supplying alternating voltages of a carrier frequency modulated at a power frequency; means varying the output voltage supplied by one of said sources in accordance with the value of a condition; demodulating means; means, including connections between said demodulating means and said sources, for producing output voltages of said power frequency in accordance with the enveopes of the voltages supplied by said sources; and means connecting said output voltages in phase opposition to produce a resultant voltage of power frequency having one phase or the other depending on the relative magnitude of said output' voltages, a device operable by voltage of said power frequency, and means energizing said device in accordance with the phase of said resultant voltage.

13. Apparatus employing voltageA alternating at a relatively high frequency for controlling the energization of a device operable by voltage of a relatively low frequency comprising, in combination: first and second sources supplying alternating voltages of a carrier frequency modulated at a power frequency; means varying the output voltage supplied by one of said sources in accordance with the value of a condition; first and second diode detectors; means, including connections between said detectors and said sources, for producing output voltages of said power frequency in accordance with the envelopes of the voltages supplied by said sources; means connecting said output voltages in phase opposition to produce a resultant voltage of power frequency having one phase or the other depending on the relative magnitudes of said output voltages; and means for energizing a device operable by voltage of said power frequency in accordance with the phase of said resultant voltage.

14. A device of the class described comprising, in combination: a source of modulated alternating voltage of a carrier frequency modulated at a power frequency, having a substantially constant amplitude; a first diode detector; means impressin g said signal voltage upon said first detector to give a first power frequency voltage output; means varying the amplitude of voltage from said source in response to a condition to provide a variable voltage; a second diode detector; means impressing said variable voltage upon said second detectorto give a second power frequency l2 voltage output; means connecting said power frequency outputs in phase opposition to give a resultant voltage of said power frequency having one phase or the other depending upon the relativ'e magnitudes of said voltage outputs: and power frequency means actuated in accordance with the phase of said resultant voltage.

15. A. device of the class described, comprising, in combination: power means supplying an alternating voltage, of a carrier frequency modulated at a power frequency, having a substantially constant amplitude; condition responsive means supplying an alternating voltage of substantially the same wave form and of varying amplitude; a plurality of demodulating means; a plurality of load circuits having in common a variable portion of a resistor; means energizing said load circuits from said power means and said condition responsive means. under the control of said demodulating means, to give voltage outputs having components of said power frequency; and power frequency means actuated in accordance with the current in one of said load circuits.

16. A device of the class described comprising, in combination: a plurality of balanceable networks; a source of complex alternating voltage having substantial components of different frequencies; means energizing a iirst of said networks from said source; asymmetrically conducting means energizing the second of said networks with components of said complex voltage derived jointly from said source and said ilrst network; condition responsive means for varylng the state of balance of said rst network; motor means operated by one of said components of said complex voltage for varying the state of balance of said second network; and means energizing said motor with said component of said complex voltage on unbalance of said second network so as to cause actuation of said motor means to rebalance said network.

17. In a device of the class described. in combination: a first source of alternating voltage of variable amplitude, including a carrier frequency component modulated at a power frequency; a second source of periodic voltage of fixed amplitude, including at least a component of said power frequency; means deriving from said second source a voltage of said power frequency related in amplitude thereto; means deriving from said iii-st source a voltage of said power irequency and of constant amplitude; means connectingl said derived voltages in phase opposition; and power frequency means responsive to inequality between said derived voltages.

WALDO H. KLIEVER.

REFERENCES CITED The following references are of record 1n the file of this patent:

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